The present invention relates to methods, devices, and kits employing a gel microstructure destruction model that incorporates an equivalent work integral function for use in conjunction with gels having transient gel microstructures.
As used herein, the term “gelled fluid” refers to fluid that forms a network of bonds (i.e., a gel microstructure) under either static or dynamic conditions. The strength of a gelled fluid relates, in part, to the corresponding gel microstructure, which is related to, inter alia, the intermolecular forces between the gelling agents (e.g., hydrogen bonding between polysaccharide molecules). However, the intermolecular forces can be relatively weak (e.g., as compared to ionic and covalent bonds). These relatively weak bonds may break when energy is put into the gel (e.g., by flowing or mixing the gel) and can reform over time as the energy input reduces or ceases. Therefore, a single gelled fluid may have a varying strength based on the history (e.g., the shear history) of the gelled fluid.
Gel strength can effect, inter alia, the magnitude of pressure increases exerted by the gelled fluid on the medium in which it is contained (e.g., a tubing, a pipe, a wellbore, a container, and the like) when flow is started. For example in a wellbore, some operations are often performed so as to maintain the wellbore pressure within the mud weight window, i.e., the area between the pore pressure and the fracture pressure of the subterranean formation, see FIG. 1. When the overbalance pressure exceeds the fracture pressure, a fracture may be induced and lost circulation may occur. Therefore, the gel strength can be a factor to take into account when performing equipment manipulations associated with a gelled fluid in a wellbore. The term “associated with” in conjunction with equipment or equipment manipulations and a gelled fluid refers to any equipment or equipment manipulations done in conjunction with a gelled fluid where the equipment or equipment manipulations are directly or indirectly affected by the strength of the gelled fluid, which does not imply physical contact. The term “overbalance pressure,” as used herein, refers to the amount of pressure in the wellbore that exceeds the pore pressure. The term “pore pressure,” as used herein, refers to the pressure of fluids in the formation. Overbalance pressure is needed to prevent reservoir fluids from entering the wellbore. The term “fracture pressure,” as used herein, refers to the pressure threshold where pressures exerted in excess of the threshold from the wellbore onto the formation will cause one or more fractures in the subterranean formation.
Because the gel microstructures are transient and of different strengths, working with a gelled fluid can be complex. For example, beginning flow of a relatively strong gel can lead to an increase in pressure, which in a subterranean operation can lead to fracturing of the subterranean formation and ultimately drilling fluid loss during subsequent operations. Therefore, beginning flow of gelled fluids having a substantially formed gel microstructure should be more gradual than a gelled fluid having minimal gel microstructure.
Typically, the API gel strength (API Recommended Procedure 10B-6, API Recommended Procedure 13B-1, the modified national adoption of ISO 10414-1, and API Recommended Procedure 13B-2, which generally provides a peak gel strength) has been used in conjunction with simple pressure drop equations to predict potential adverse occurrences (e.g., static peak overpressures) during the implementation of gelled fluids. Use of such a calculation method does not take into account the dynamic nature of the gel microstructure. Consequently, these calculation methods fall short in at least two ways (1) predicting adverse occurrences in a gelled fluid having a flow history and (2) inefficient equipment manipulations associated with a gelled fluid having a partially formed gel microstructure.
For example relative to the second point, inefficient equipment manipulation may occur using the simple pressure drop equations with a peak gel strength in operations that require a series of equipment manipulations where the gel microstructure of a gelled fluid undergoes break down and formation repeatedly, e.g., pipe tripping in a wellbore. By way of a nonlimiting example, pipe tripping may involve adding 90 foot lengths of pipe to a pipe string where the pipe length is added, then the pipe string is moved down the wellbore, then the pipe string is stopped so as to add another pipe length, then the pipe string is moved down the wellbore, and so on until depths of, in some cases, greater than 20,000 feet is achieved. In some instances, it can take about 30 seconds to move the pipe string down the wellbore for each pipe length and 5 minutes to attach another pipe length. Movement of the pipe string down the wellbore yields a decrease in the stress response of the gelled fluid because the microstructure of the gelled fluid is being broken. The stop in movement during attachment of another pipe length allows for the stress response of the gelled fluid to increase because the microstructure of the gelled fluid is being reformed.
FIG. 2A provides three graphs illustrating (a) the wellbore pressure as a function of time, (b) the running speed of the pipe (i.e., the speed of axial movement of the pipe in wellbore) as a function of time, and (c) the stress response of the gelled fluid as a function of time. Further, in (a) the wellbore pressure illustrative graph, the fracture pressure is indicated along with a maximum desired wellbore pressure. The area between the plot of wellbore pressure as a function of time and the maximum desired wellbore pressure is a measure of the efficiency of the operation, as illustrated in FIG. 2A(a). That is, the more area between the plot and the maximum desired wellbore pressure, the less efficient the operation because in pipe tripping operations the speed of the operation is heavily influenced by the need to stay below the fracture pressure of the formation, as is generally the case in many wellbore operations. Therefore, using current calculation methods (i.e., simple pressure drop equations with a peak gel strength) that do not take into account the shear history of gelled fluids, as illustrated in FIG. 2A(c), the same procedure for running the pipe is performed for each length of pipe, as illustrated in FIG. 2A(b). However, if the gelled fluid does not regain a gelled state stress response (Δm) in the time required to attach another pipe length to the pipe string, then using the same procedure for running the second, third, and so on pipe lengths is inefficient in that the wellbore pressure is maintained at levels far below the maximum desired wellbore pressure, as illustrated in the area between the plot and the maximum desired wellbore pressure of FIG. 2A(a).
Approaches to model the transient nature of the gel microstructure fall short in the field because they often require detailed rheological measurements. The acquisition of detailed rheological measurements, e.g., at a well site, can be time consuming and difficult to obtain in some cases where a more specialized rheological measurement is needed. Further, the application of these detailed rheological measurements to yield useable information may be cumbersome or take more time than they save.
Therefore, a need exists for integrating the transient nature of gelled fluids into the methods and apparatuses relating to gelled fluids, e.g., in relation to subterranean operations.